1. Field of the Invention
The invention disclosed in the present specification relates to a substrate on which a thin film semiconductor device typified by a thin film transistor is formed and to a method for fabricating the substrate. The invention also relates to a thin film transistor formed on an insulating substrate such as a glass substrate and a quartz substrate and to a method for fabricating the same.
2. Description of Related Art
Hitherto, there has been known a technology for fabricating a thin film transistor (hereinafter referred to as a TFT) on a glass or quartz substrate. This technology is required to fabricate an active matrix type liquid crystal display.
Presently, the mainstream thereof is a-Si TFT using an amorphous silicon film. However, the a-SiTFT is applied only in constructing an active matrix circuit because its operating speed is slow.
A structure in which a peripheral driving circuit and other circuits, beside the active matrix circuit, are integrated on one glass substrate is being proposed lately. This structure is called a system-on-panel. The structure called the system-on-panel is a configuration required in miniaturizing and lightening an apparatus comprising the liquid crystal display. It is also useful to integrate the circuits having the various functions on one substrate in simplifying fabrication steps and operation checks.
When a TFT is fabricated on the insulating substrate such as the glass substrate or the quartz substrate however, its characteristic has been questionably low and varied. When the characteristic is low, characteristic of a circuit comprising such a TFT is also degraded. When the characteristic varies greatly, characteristic of a circuit comprising such a TFT varies and is degraded.
The grade of the characteristic is related mainly to physical properties of a semiconductor film to be used. The characteristic of the TFT may be enhanced by using a silicon film having high crystallinity.
Meanwhile, the variation and the instability of the characteristic of the TFT are considered to be caused by:
1) instability of process; and
2) electrical instability of a thin film semiconductor obtained.
The variation and the instability of the characteristic of the TFT of an active matrix liquid crystal display are considered to be caused by impurity mixed in steps for fabricating the TFT because:
1) a glass or quartz substrate containing impurity in high concentration as compared to a semiconductor substrate is used as a substrate in the active matrix type liquid crystal display; and
2) the size of a deposition system and a conveyor system increases in correspondence to the substrate having a relatively large area.
For instance, impurity mixed into a semiconductor film composing the TFT in steps for depositing it causes the variation and the instability of the TFT characteristic.
Then, the inventor et. al. have measured film quality and impurity of a gate insulating film which has a large influence on the characteristic of the TFT to study the above-mentioned relationship between the characteristic of the TFT and the variation.
FIG. 11 shows data concerning to impurity existing at the interface between a gate insulating film and a gate electrode of a TFT formed on a Corning 1737 glass substrate. This data is a result of the measurement carried out by means of EDX (energy distributed X-ray micro-analysis). EDX senses elements existing in the order of more than 0.1%. Accordingly, elements detected by the EDX analysis means to exist in the order of more than 0.1% (percentage of number of elements).
This sample is what a silicon oxide film deposited by means of plasma CVD is used as the gate insulating film and aluminum deposited by means of sputtering is used as the gate electrode.
Accordingly, peaks of silicon (Si), oxygen (O) and aluminum (Al) are seen in FIG. 11. However, peaks of trace amounts of barium (Ba) and calcium (Ca) are also seen.
Although the vertical axis of the measured value shown in FIG. 11 is not what reflects the percentage of elements accurately, it shows a relative relationship of the density of their existence.
While counted numbers of barium and calcium are not so large as compared to those of aluminum and silicon in FIG. 11, their density is considered to be high when their electrical influence is considered (they exist at least more than 0.1%).
Barium and calcium are liable to be ionized. Accordingly, such elements existing at the interface between the gate insulating film and the gate electrode with concentration of more than 0.1% may become a significant factor of destabilizing the operation of the TFT.
FIG. 12 shows a result of analysis of the Corning 1737 glass substrate utilized as a substrate implemented by the same measuring method with that shown in FIG. 11.
As it is apparent from FIG. 12, this glass substrate contains relatively high concentration of barium and calcium. It can be considered from this fact that barium and calcium shown in FIG. 11 are what have turned around from the glass substrate used as described above.
This turn-around of the impurity from the glass substrate is considered to have occurred when the gate eletrode is deposited, the substrate is sputtered and the impurity flies within the ambient atmosphere.
Further, quartz is required to use as a substrate in implementing heat treatment in a temperature as high as 800° C. and 900° C. However, although a substrate composing a semiconductor device is required to have high impurity as described above, such a quartz substrate is expensive in general.
A number of ranks exists among quartz substrates and a quartz substrate in the lower rank is inexpensive. However, a quartz substrate in the lower rank contains high concentration of OH group and the OH group affects the operation of the semiconductor device fabricated on the substrate. For instance, it becomes a factor of shifting a threshold value of the TFT on the minus side.
In general, the OH group within the quartz substrate is a factor of destabilizing the operation of the semiconductor device fabricated on the substrate and of varying the characteristic of the device.
According to the measurement carried out by the inventors et. al. of the present invention, concentration of OH within the low grade quartz substrate has been higher than that of a higher grade quartz substrate by more than 15 times.
There has been known a crystal grass (called also as a ceramic glass) as an inexpensive glass substrate having a high heat resistance and a distortion point of 700° C. or more. However, because the crystallized glass substrate also has various components, it is feared that impurity may diffuse from the substrate in the process for manufacturing the semiconductor device.
Accordingly, based on the recognition described above, it is an object of the invention disclosed in the present specification to construct a substrate for a semiconductor device which allows a stable semiconductor device having no variation of characteristic to be obtained by preventing impurity from turning around to the semiconductor device from a glass or quartz substrate (or another adequate substrate), and to provide a method for fabricating such a substrate.
It is another object of the invention to provide a semiconductor device whose variation of characteristic and instability have been eliminated and to a method for fabricating such a semiconductor device.